1. Field of the Invention
The present invention relates generally to an electrophotographic printing apparatus and more particularyly to a device for the removal of particles adhering electrostatically to an insulating surface in a two-color electrostatographic machine.
2. Description of the Prior Art
A novel and unique two color electrostatic copying machine is disclosed in the U.S. Pat. Nos. 4,189,224 and 4,250,239, and in the U.S. Pat. Nos. 4,335,194 and 4,310,610 assigned to Ricoh Company, Ltd. The present invention constitutes an improvement to the cleaning device in the basic two-color copying machine.
A two color copying machine disclosed in the aforesaid patents comprises a photoconductive drum which is formed with a grounded, electrically conductive core, and inner and outer photoconductive layers of different spectral sensitivities. Around the drum, the following units are arranged:
First and second chargers applying electrostatic charges of opposite polarities to the drum to form a stratified charge pattern thereon. Exposure unit for radiating a light image to form an electrostatic image according to respective colors on the photoconductive layers of the drum. First and second developing units for applying positively charged toner of one color and negatively charged toner of the other color respectively to the drum to form a two-color toner image thereon. Pre-charger unit for converting the toner image having opposite polarities to one uniform polarity. Feed means for feeding a copy sheet into engagement with the drum at the same surface speed as that of the drum to transfer the toner image to said copy sheet. Transfer charger for applying the charge to the back of the copy sheet to facilitate the toner image transfer. Discharge unit for discharging the drum and cleaning unit for removing any residual toner particles therefrom prior to the next copying operation.
In such a copying machine, the cleaning element typically comprises a rotatable brush with a core having a plurality of fibers extending radially outwardly therefrom. The brush sweepingly engages the photoconductive drum surface to remove the residual toner particles therefrom. Such brush fibers are formed of soft hairs, for example, of a bear or a beaver, and the brush fibers are made from a single kind of material. Consequently, while the brush can effectively remove the toner particles of one polarity, it is not always suitable to remove the toner particles of the other polarity with a result that residual toner particles left on the photoconductive drum can affect the next copying operation.
The above-mentioned problem results from the electrostatic images formed in opposite polarities in the stratified charge pattern on the surface of each photoconductive layer corresponding to respective colors. Although, prior to cleaning, pre-charge, transfer charge, and discharge are applied to the surface of the drum, such charging is not always sufficient to enable the above-mentioned cleaning unit to clean the surface of the drum thoroughly
It is especially difficult to remove the residual toner particles corresponding to the electrostatic image on the inner photoconductive layer.
In order to solve this problem, two brush fiber rollers wherein each fiber is charged with opposite polarities may be used, but this is still undesirable because it tends to make the cleaning unit bigger.
The present invention has been made to solve the aforesaid drawbacks in a two-color copying machine.